A detent escapement answering the above description has already been proposed and disclosed in the old Swiss Patent No. CH-3299 in the name of Emile James. FIG. 1 of the present description shows a detent-lever pivoted at one of its ends in accordance with a conventional design of this type of escapement.
The balance pin 40 carries a large roller 41, a first small roller 42 having a notch 43 and a second roller 44 carrying an actuating finger 45. The detent-lever 46 is pivoted on a pin 47. It carries a locking pallet-stone 48 a pin 49, a beak 50 and a strip spring 51. The detent-lever 46 is returned to the rest position by a spiral return spring 53.
At the moment when the actuating finger 45 raises the detent-lever 46 with the assistance of the strip spring 51, the beak 50 penetrates the notch 43 at the same time that the escapement wheel 52 moves forward one step. During the additional arc, the beak 50 is released from the notch and is in proximity to the circular periphery 54 of the first small roller 42 in the position shown in the Figure.
This arrangement has the advantage of preventing a tooth of the wheel 52 from leaving the locking pallet stone 48 when the timepiece receives a shock. Indeed, at that moment, the beak 50 abuts for a brief moment against the circular periphery 54 of the first small roller 42, which stops the detent-lever 46 which is immediately returned to the rest position by the spiral return spring 53.
The foregoing identifies a weakness affecting the detent escapement, namely that it is very sensitive to shocks, thus this escapement is reserved especially for chronometers of large dimensions or marine chronometers that are not mechanically stressed, said escapement having the reputation of not being suited to wristwatches. In order to prevent the locking pallet-stone to release itself from the escapement wheel inadvertently, it has already been proposed, in addition to the above-mentioned solution, to fit the blocking member with an additional finger arranged to prevent the rotation of the escapement wheel in the case where such an inadvertent release occurs (see for example the work by Huguenin, Guye and Gauchat, Neuchâtel 1975, FIG. 17-5) entitled “Echappements et Moteurs pas à pas”.
Referring again to the afore-cited Swiss Patent, one note however that removal of the beak 50 from the notch 43 is only possible owing to the spiral spring 53, which exerts a return force on the detent-lever 46. Indeed, the notch 43 carries almost radial sheer flanks preventing any removal of the beak which might be caused simply by rotation of the roller itself.
One could thus omit the return spring if one would shape the notch and the beak which penetrates therein in such a way that the return effect of the detent is caused by the rotation itself of the roller. This is one object the present invention.